scoru: add carbonated function cost for serializing internal inbox message and deserializing outbox message

Context

There were no cost functions for serialization of an internal message inbox or deserialization an outbox message

fix #2951 (closed)

Manually testing the MR

No way to test it right now but a following MR should use those functions

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR